Titanium and titanium alloys have a number of outstanding properties, including high strength-to-weight ratios and excellent resistances to chemical attack, that make them desirable materials for many component applications in the aerospace and biomedical industries. The usefulness of titanium and titanium alloys for component or device design has been limited by the lack of a viable commercial hermetic sealing technology. Titanium undergoes deleterious reactions with commercial silicate-based sealing glasses to form an interfacial silicide reaction product that produces a very weak bond.
Boroaluminate glasses as disclosed in U.S. Pat. No. 5,104,738 to Brow et al are potential candidates for titanium glass-to-metal seals since they have a coefficient of thermal expansion close to that of titanium, and provide glass-to-metal seals with a mechanical strength exceeding that of seals formed from the commercial silicate sealing glasses. A drawback to the boroaluminate glasses, however, is their relatively poor aqueous durabilities which limit, in particular, both in vivo applications (e.g. for implanted batteries, heart pacemakers, defibrillators, pumps or the like) and applications requiring prolonged contact with moisture or water.
Thus, there is a need for improved titanium sealing-glass compositions having an aqueous durability sufficiently high to permit the formation of glass-to-metal seals that are resistant to moisture, water or body fluids (e.g. for in vivo use). There is a further need for improved titanium sealing-glass compositions having favorable viscosity characteristics for forming glass-to-metal seals with titanium or titanium alloys at a sealing temperature below about 900.degree. C. and with little or no tendency for crystallization.
An advantage of the titanium sealing-glass compositions according to the present invention is that a glass-to-metal seal can be formed by a sealing-glass body in contact with a titanium or titanium alloy to provide an aqueous durability that exceeds that of boroaluminate glasses by up to an order of magnitude or more.
Another advantage of the present invention is that the titanium sealing-glass compositions according to the present invention have a glass transition temperature less than about 600.degree. C. so that the glasses can be sealed at a temperature below an allotropic .alpha.-.beta. phase transition temperature of pure titanium near 882.degree. C.
A further advantage is that the titanium sealing-glasses of the present invention have favorable viscosity characteristics at a preferred sealing temperature of about 700.degree.-800.degree. C. for fusing to titanium or a titanium alloy without any need for weights or the like (e.g. weighted graphite fixtures placed above the sealing glasses to promote a flow of the sealing glasses) to aid in seal formation.
Still another advantage is that the titanium sealing-glass compositions of the present invention can be used for forming titanium glass-to-metal seals for in vivo applications including implantable batteries, pacemakers, defibrillators and pumps.
These and other advantages of the titanium sealing glasses of the present invention will become evident to those skilled in the art.